Tatl is Link's fairy companion and the sister of Tael. As a fairy companion, Tatl is not as informative as Navi from Ocarina of Time and has somewhat of an attitude, however, her intentions are good. At first, she is cold and callous towards Link, but eventually grows fond of him, much like Midna. Unlike Navi, who tells Link about enemies in an encyclopedic manner, Tatl often interjects her own opinions into her information (when telling Link about the Stalchildren, she says that they are cursed and adds "What are you going to do about it?"). She also seems surprised about Link's lack of knowledge when asked about enemies that appeared Ocarina of Time as well as Majora's Mask.
Source: Wikipedia, "Characters in The Legend of Zelda series," available under the CC-BY-SA License.